error: Functions to report warnings and informational messages

Add warn_report(), warn_vreport() for reporting warnings, and
info_report(), info_vreport() for informational messages.

These are implemented them with a helper function factored out of
error_vreport(), suitably generalized. This patch makes no changes
to the output of the original error_report() function.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <c89e9980019f296ec9aa38d7689ac4d5c369296d.1499866456.git.alistair.francis@xilinx.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>